Our motorhome is a 2005 24 foot Itasca Class C weighing in at only around 11,800 lbs., but built on the heavier E450 Ford chassis instead of the usual E350.
This means that we're under-loading the chassis when compared to it's maximum rating. However, even at that I consider that the chassis is actually "hauling" all the time. As such, we travel in our Class C with Tow/Haul mode always engaged.
Even with Tow/Haul mode engaged, when cruising on the highway the transmission goes to, and stays in, overdrive just fine on the level stretches. I also really like the more spirited response I get when driving in cities and towns with Tow/Haul mode engaged - faster pulling away from the lights, more positive stops, and in general just a better "in control" feeling.